1. Field of the Invention
The present invention relates to a communication apparatus, a communication method, and a communication system for making a media access control based on carrier sense information contained in a physical layer and carrier sense information contained in a MAC layer.
2. Description of the Related Art
A Media Access Control (MAC) is to determine how a plurality of communication apparatuses making communication by sharing the same media utilize media to transmit communication data. In the case where two or more communication apparatuses transmit communication data by utilizing the same media at the same time, there can occur an event (collision) that a receiving communication apparatus cannot isolate communication data. On the other hand, although a communication apparatus having a transmitting request exists, there can occur an event that a medium is not utilized by any communication apparatus. In order to minimize such events, a media access control technique is used to control an access from a communication apparatus to a medium.
However, in particular, in wireless communication, it is difficult to monitor transmission data at the same time when a communication apparatus transmits data. Therefore, a media access control (MAC) which does not presume detection of collision is required. IEEE 802.11 which is a typical technical standard of a wireless LAN uses a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). In the CSMA/CA of the IEEE 802.11, at a heater of a MAC frame, a Duration is set until a series of sequences consisting of exchange of one or more frames that follow the frame have terminated. In this duration, a communication apparatus which does not relates to the sequences and which does not have a transmission privilege determines a virtual occupancy state of a medium, thereby waiting for transmission. Therefore, an occurrence of a collision is avoided. On the other hand, a communication apparatus having a transmission privilege using the sequences recognizes that no media is used to expect a duration in which a physical medium is actually occupied. The IEEE 802.11 defines that a media access is controlled by determining a state of a medium in accordance with a combination between a virtual carrier sense in such a MAC layer and a physical carrier sense in a physical layer.
Up to now, the IEEE 802.11 using the CAMA/CA has promoted a higher communication speed by mainly changing a physical layer protocol. With respect to a 2.4 GHz bandwidth, the IEEE 802.11 (1997, 2 Mbps) is changed to an IEEE 802.11b (1999, 11 Mbps), and the IEEE 802.1b is changed to an IEEE 802.11g (2003, 54 Mbps). With respect to a 5 GHz bandwidth, only an IEEE 802.11a (1999, 54 Mbps) is now provided as standard. In order to define a standard for a higher processing speed in both of the 2.4 GHz bandwidth and the 5 GHz bandwidth, an IEEE 802.11 TGn (Task Group n) has already been established.
If a frequency spectrum identical to that in the existing standard is used in achievement of a higher communication speed, a communication apparatus newly provided enables coexistence with a communication apparatus that follows the existing standard. It is preferable that backward compatibility be maintained. Therefore, it is considered to be basically good that a protocol of a MAC layer follows the CSMA/CA which matches the existing standard. In this case, it is necessary to ensure that a time-related parameter associated with the existing standard, for example, an interframe time interval (IPS: Interframe Space) or a back-off duration conforms to the existing standard.
Here, even if a higher data rate has been successfully achieved with respect to a physical layer, there is a problem that a substantial communication throughput cannot be improved. That is, in the case where a higher data rate of the physical layer has been achieved, a format of a PHY frame is no longer effective. It is considered that an overhead caused by this problem inhibits improvement of throughput. In the PHY frame, a time-related parameter according to the CSMA/CA is fixedly associated with the MAC frame. In addition, a PHY frame header is required for each MAC frame.
One method for reducing an overhead to improve a throughput includes Block ACK introduced in the latest draft IEEE 802.11e draft 5.0 (strengthening QoS of the IEEE 802.11). By using this method, a plurality of MAC frames can be continuously transmitted without back-off. Thus, an amount of back-off can be reduced, but a header of a physical layer cannot be reduced. In addition, according to an aggregation introduced in accordance with an earlier draft IEEE 802.11e, both an amount of back-off and a physical layer header can be reduced. However, a length of a frame of a physical layer including the MAC frame cannot be set to about 4 Kbytes or more because of a restriction on a conventional physical layer. Therefore, large restriction applies to improvement of efficiency. Even if a frame of a physical layer can be increased in length, there occurs a problem that error tolerance is lowered.